Petroleum production methods currently are utilizing high pressure nitrogen injection to maintain well head pressure for enhanced oil and gas recovery. As nitrogen is injected, the natural gas from the well containing methane and associated hydrocarbon liquids also contains nitrogen which increases in amount over the life of the nitrogen injection project. For this reason. natural gas containing nitrogen must be separated to reject the nitrogen and form purified natural gas feedstocks suitable for utilization as fuel or chemical feedstocks.
U.S. Pat. No. 3,797,261 discloses the separation of natural gas containing nitrogen into a low-nitrogen fraction and a high nitrogen fraction by distillation in a single distillation column by expanding the high-nitrogen fraction with the performance of work and using the resulting refrigeration to condense vapor in the upper section of the column while additional reflux is provided by vaporizing a recycle medium in heat exchange relation with vapor in the column. The high-nitrogen mixture, having been expanded, is exhausted at atmospheric pressure.
U.S. Pat. No. 4,411,677 discloses a process for rejecting nitrogen from a natural gas feed containing nitrogen over a broad range of compositions, under elevated pressure using a single distillation column, and a closed loop methane heat pump which reboils and refluxes the column. An intermediate reflux condenser is cooled by both the heat pump and overhead nitrogen stream of the column. A mixed cryogenic refrigerant can be used in the heat pump as an alternative to the methane heat pump medium. The process provides a high methane recovery over the entire feed range and a pressurized nitrogen product stream that can be used for recycling and reinjection into an oil or gas well to improve head pressure.
U.S. Pat. No. 4,504,295 discloses a process for the recovery of methane, nitrogen and natural gas liquids from a natural gas feed stream wherein the recovery can be made at high pressure by integration of a nitrogen rejection stage including a heat pump driven distillation column and a natural gas liquid stage. Nitrogen can be rejected over a wide range of nitrogen concentration of the feed stream.